1. Field of the Invention
The present invention relates to double solenoid valves in which a valve member can be switched between two switch positions in a valve hole of a valve body, and more specifically to double solenoid valves with a detent mechanism in which the detent mechanism prevents the valve member from being displaced for some reasons from the switch positions, thereby stabilizing an operation of the valve member.
2. Description of the Related Art
Double solenoid valves in which a valve member having a peripheral seal element is configured to be switched between two positions in a valve hole of a valve body in the axis direction are commonly known. Since such double solenoid valves, especially those which include the valve member made from a metal seal spool, have small friction resistance between the valve member and the inner surface of the valve hole, a detent mechanism that holds the valve member in the switch positions needs to be provided in order to stabilize holding of the valve member in the switch positions.
It is not only when the valve member is made from a metal seal spool that the valve member needs to be held in the switch positions to stabilize the operation, but also, for example, when the valve has a seal member having rubber elasticity that protrudes from the periphery of the valve member so as to allow the seal member to be in frictional contact with the inner surface of the valve hole. In this case, when the solenoid is de-energized after the position of the valve member is switched to hold the position of the seal member by frictional contact with the inner surface of the valve hole, the valve member may be displaced from the switched position due to an external force of impact or the like. Accordingly, in order to prevent the valve member from being unintentionally displaced and to comply with the European safety standard (ISO 13849-1) category 1, the above detent mechanism needs to be provided.
One example of the above detent mechanism is disclosed in Patent Literature 1, Japanese Unexamined Patent Application Publication No. 10-22123. Generally, in the detent mechanism, a groove is disposed on the outer periphery of the spool or recesses having a concave spherical shape are disposed at a plurality of positions equally spaced on the outer periphery of the spool, while steel balls are disposed on the inner periphery of the valve hole at a holding position for positioning which corresponds to the groove or the recesses on the spool in a manner capable of being protruded by a pressure applied by a coil spring on the back of the steel ball so that the protruding portion of the steel ball is fit in the groove or recesses to stably hold the spool in the switch positions.
However, in the above detent mechanism, the increased number of components is necessary for mounting the detent mechanism, and in addition to that, a holding structure for the steel balls which elastically protrude to the inner surface of the valve hole needs to be provided on the valve body and the circumferential groove or the recess needs to be formed on the outer periphery of the spool, which causes the manufacture cost to be significantly increased.